Multi Stage Underwater Sensor Localization using Mobile Beacons ∗ Melike Erol, † Luiz F. M.Vieira, ‡ Antonio Caruso, ‡ Francesco Paparella, † Mario Gerla, ∗ Sema Oktug ∗ Istanbul Technical University, Computer Engineering Department, Istanbul, Turkey † UCLA Computer Science Department, Los Angeles, California ‡ Mathematics Department, University of Salento, Lecce, Italia {melike.erol,oktug}@itu.edu.tr {antonio.caruso,francesco.paparella}@unile.it, {luiz,gerla}@cs.ucla.edu Abstract—Underwater Sensor Networks (USN) are used for harsh oceanographic missions where human operation is dan- gerous or impossible. Localization is essential for USNs. It is required for data tagging, node tracking and position-based routing algorithms. Localization is challenging because Global Positioning System (GPS) is not available in underwater; at the same time, existing GPS-less schemes based on fixed landmarks have high communication cost. Such cost is critical in Mobile Underwater Sensor Networks (MUSN), since sensor nodes drift with the ocean currents, thus requiring continuous refresh. In this paper, we propose a multi-stage localization scheme using mobile beacons. The beacons periodically ascent and descent in the water column. When they resurface, they receive new GPS coordinates. Then, they dive to the level of the underwater sensors to advertise these coordinates. In turn, localized sensors become proxy beacons and propagate their own coordinates, etc. This iterative, multi-stage localization is the major innovation of this paper. The goal is to localize the nodes with the smallest number of beacons using proxies instead, yet achieving an adequate accuracy. The major benefit is the reduction in operating costs. Mobility is a critical factor in determining performance. In this paper, performance (i.e., the percentage of localized nodes during a cycle, accuracy, delay and communication cost) is tested in a simulation scenario based on a realistic mobility model. The “Meandering Current Mobility with Surface Effect” (MCM-SE) model - a composite model combining surface and subsurface currents. I. I NTRODUCTION Sensor networks are becoming highly involved in our daily lives as they continuously collect data and monitor the surrounding environment. Raw sensor data are meaningful with the context, i.e., the knowledge of where and when the data is collected. This is known as data tagging. In addition, localization is required for node tracking, target detection and position-based routing algorithms. Sensor networks that operate outdoors are able to benefit from the GPS with some extra cost. Indoor, underground or underwater sensor networks need some specialized solutions for localization. Underwater Sensor Networks (USNs) can improve ocean exploration, allowing a list of new applications that are presently not possible or very costly to perform, including: oceanographic data collection, ecological applications (e.g. pollution, water quality and biological monitoring), public safety (e.g. disaster prevention, seismic and tsunami monitor- ing), military underwater surveillance, industrial (offshore ex- ploration), etc. However, before USNs become commercially available or widely used, the networking of sensor nodes in underwater has to be addressed. Medium access and packet forwarding are still active research areas in USNs [1]–[6]. Localization is another challenging task. The use of GPS is restricted to surface nodes because the GPS signal does not propagate through water. Alternative GPS-less positioning schemes have been proposed for terrestrial sensor networks but they have to be revised due to acoustic channel properties. The acoustic channel has low bandwidth, high propagation delay and high bit error rate. Therefore, localization protocols need to work with minimum possible message exchange. This is also dictated by the limited battery power of the sensor nodes and the difficulty of recharging or replacing batteries of the underwater nodes. In Mobile Underwater Sensor Net- works (MUSNs), the mobility of free-floating nodes brings up another challenge in localization. In this paper, we address the localization issue for MUSNs. We propose a multi-stage localization protocol using mobile beacons. Mobile beacons receive absolute time and location in- formation from GPS when they float on the surface. Then, they periodically descent to distribute their coordinates. They are able to dive and rise with volume expansion therefore we name them as Dive and Rise (DNR) beacons. In addition to DNR beacons, an iterative, multi-stage localization is employed. The already localized nodes become active beacons and distribute their coordinates. Our protocol aims to maximize the number of localized nodes while keeping the error, communication overhead and the delay low. In simulating a mobile network, the ability of the mobil- ity model to capture the real life observations is of major importance. Here, we use the “Meandering Current Mobility with Surface Effect” (MCM-SE) model. The MCM was first suggested by physical oceanographers as a simple model for lagrangian studies of western boundary currents [7] and it is applied to underwater sensor networks in [8]. The MCM describes a sub-surface, jet-like current meandering around recirculating vortices. In this work, we model the surface mobility with a stochastic process superimposed to the MCM. We study the performance of our localization protocol when The Second International Conference on Sensor Technologies and Applications 978-0-7695-3330-8/08 $25.00 © 2008 IEEE DOI 10.1109/SENSORCOMM.2008.32 717 The Second International Conference on Sensor Technologies and Applications 978-0-7695-3330-8/08 $25.00 © 2008 IEEE DOI 10.1109/SENSORCOMM.2008.32 710